Chromosome heteroploidy is responsible for a substantial segment of birth defects and reproductive inefficiency. Each year in the United States 15,000 infants with a chromosome abnormality are born, and there are 175,000 spontaneous abortions of chromosomally abnormal fetuses. Using hybrid, inbred and random bred mouse strains we will investigate (1) whether the relationship of increased maternal age to hyperdiploidy in mouse fetuses reflects a relationship existing in mature mouse ova or is secondary to a decreased ability to reject hyperdiploid embryos, and (2) when the meiotic error or errors occur that cause heteroploidy in mouse embryos. For the latter, oocytes matured in vivo and in vitro to metaphase I and metaphase II and pronuclear zygotes will be used. Finally, the effect of such variables as maternal age, the recent use of birth control pills, and the method of inducing oocyte maturation on the incidence of heteroploidy in maturing human ova will be examined. Human ova in the germinal vesicle stage for in vitro maturation will be obtained by mincing the ovaries and in vivo stages will be obtained by giving patients Pergonal prior to elective gynecologic surgery. A Giemsa banding technique will be used to enable us to identify specific chromosome pairs and more accurately describe heteroploid oocytes.